Spread spectrum clock generator

ABSTRACT

A spread spectrum clock generator includes a non-volatile memory to store control codes corresponding to a predetermined delay. A delay circuit receives a control code having a predetermined number of bits that determine a delay to apply to a fixed clock signal a period of time. The delay mitigates the electromagnetic interference caused by a periodic clock signal.

This application claims priority from Korean patent application number 2003-62863 filed Sep. 8, 2003 that we incorporate herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a spread spectrum clock generator and a method of generating a spread spectrum clock.

2. Description of the Related Art

A clock generator, such as 10 in FIG. 1, generally comprises a clock source 100 and a phase-locked loop (PLL) 102. This generator 10 generates a system clock usually having square waves and a 50% duty cycle. The system clock is then used in many different types of systems, such as a memory system including a memory module 14 and a memory controller 12.

System clocks such as this can be a source of unwanted electromagnetic interference (EMI). EMI can cause problems in electronic circuits, as it interferes with signal transmission. As technology improves, circuits can operate faster, requiring faster clocks that in turn generate more EMI. One technique to alleviate EMI is to use spread spectrum clock generators (SSCG). These clocks are referred to as spread spectrum in that their frequency is spread out over different frequencies, avoiding the energy peaks at clock edges. In some instances, SSCGs are implemented using PLLs as shown in the US patents described below. PLLs vary the voltage to a voltage controlled oscillator (VCO), causing varying delays in the clock.

Examples of this approach are shown in U.S. Pat. No. 5,631,920, issued May 20, 1997; U.S. Pat. No. 5,692,507, issued Sep. 18, 2001; and U.S. Pat. No. 6,351,485, issued Feb. 26, 2002. The use of a PLL typically allows a clock cycle to be switched between two frequency limits, adjusting the clock frequency back and forth between them. This approach may be somewhat limited, as it only allows for two fixed frequencies to be used, and does not allow programmable control.

Another approach is shown in U.S. Pat. No. 6,501,307, issued Dec. 31, 2002. As shown in FIG. 2, this approach has 2 capacitors used as loads switched by counter-sequencer 20, clocked by fixed clock FCLK. The counter-sequencer 20 drives first control signal CTL1 to the gate of load-switching transistor 22, and second control signal CTL2 to the gate of second load-switching transistor 24. When CTL1 is high, capacitor 26 has to be charged and discharged by input buffer 28 before the logic threshold of output buffer 30 is reached, thus delaying the clock edges. When CTL2 is high, capacitor 32 has to be charged and discharged by input buffer 28 before the logic threshold of output buffer 30 is reached, thus also delaying the clock edges. When both CTL1 and CTL2 are high, both capacitors have to be charged, further delaying the clock edges. However, these loads cannot be changed linearly to adjust the clock frequency as needed.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the invention will become more readily apparent from the detailed description of an embodiment that references the following drawings.

FIG. 1 shows a prior art embodiment of a memory system.

FIG. 2 shows a prior art embodiment of a spread spectrum clock generator.

FIG. 3 shows a signal diagram of energy pulses associated with clock generators.

FIG. 4 shows an embodiment of memory system according to this invention.

FIG. 5 shows an embodiment of a memory system employing a spread spectrum clock generator according to this invention.

FIG. 6 shows an alternative embodiment of a memory system employing a spread spectrum clock generator according to this invention.

FIG. 7 shows an embodiment of a spread spectrum clock generator according to this invention.

FIG. 8 a-8 b show alternative embodiments of a delay circuit according to this invention.

FIG. 9 shows an embodiment of a control circuit for a spread spectrum clock generator according to this invention.

FIG. 10 shows an embodiment of an address generator according to this invention.

FIG. 11 shows a timing diagram for a spread spectrum clock generator according to this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 3 shows the basis of the problem with nonmodulated clock signals. The energy spike from a nonmodulator clock may have an amplitude of two to eighteen dB beyond that of a modulated, or spread spectrum, clock signal. This difference causes a much higher level of EMI that can have negative effects on electronic components and systems such as memory systems. Examples discussed herein may rely upon memory system components and methods, but are merely intended as examples, and it must be understood that application of embodiments of the invention are not limited to memory systems.

An example of such a system is shown in FIG. 4. A clock generator 40 generates a fixed frequency clock, FCLK, that is used by the spread spectrum clock generator (SSCG) 42. SSCG 42 produces a spread spectrum clock and is in turn used by electronic devices 44 a through 44 n. In a memory system, the devices 44 a-44 n may be memory banks or memory module or memory device or registers used to store data.

Alternative embodiments of a memory system employing a spread spectrum clock generator are shown in FIGS. 5 and 6. In FIG. 5, the clock generator 90 includes fixed frequency clock 900 and a phase locked loop 902. The memory module 94 includes individual memory modules and the spread spectrum clock generator 904.

The SSCG is shown in a more detailed embodiment in FIG. 7. In this embodiment, the SSCG 42 has a control circuit 50, a programmable delay circuit 52 and a register circuit 54. The register circuit 54 holds control codes that program the delay circuit 52. The control circuit 50 provides the addresses to the register circuit 54, which in turn provides the control codes to the delay circuit. This allows the delay to alter a delay period applied to a fixed clock FCLK, changing the frequency of the clock to alleviate the EMI of a periodic clock.

The programmable delay may be implemented by one of many sets of delaying components. Two examples are provided in FIGS. 8 a and 8 b, but it must be noted that these are merely examples of delay components. Embodiments of the invention generally provide components that can be selected by the control codes provided from the register circuit, allowing precise control of the delay in a spread spectrum clock generator. In the example of FIG. 8 a, the delay elements are oppositely arranged capacitors, such as NMOS and PMOS capacitances. The fixed clock, FCLK, is buffered by inverting input buffer 60. If the FCLK signal is high, the inverted signal is low. This causes a low signal to be at one terminal of the PMOS capacitances 62 a, 62 b and 62 c. If the control code for a particular component is low, the PMOS capacitance for that component will provide 100% of the capacitance, causing a delay equal to the charging time of the component.

For example, if the control code CO1 is low, the capacitor 62 a provides 100% of the capacitance that will need to charge before the signal can pass to the output inverter 66. If the control code CO1 is high, the capacitor 62 a provides substantially ⅓ of the capacitance that would need to charge before the signal passes to the output inverter 66.

If the clock signal FCLK is low, the output of the inverter 60 is high. This causes the NMOS capacitors 64 a-64 c to be the line loads for the signal prior to reaching the output inverter 66. In this manner, the amount of delay can be programmed by the control codes, in conjunction with the input clock signal FCLK.

Another example of a delay circuit is shown in FIG. 8 b. Each delay component in this embodiment has an access transistor such as 72 a and a capacitor such as 74 a. When the control code for a particular component is high, the access transistor turns on and the capacitor will charge, causing a delay. For example, if the control code CO1 is high, the transistor 72 a turns on and capacitor 74 a will charge. This causes a delay in the transmission of the signal from the input inverter buffer 70 to the output inverter buffer 76. Each additional capacitor that turns on will cause the capacitors to charge, thereby increasing the delay.

The capacitors of FIGS. 8 a and 8 b may all have the same value, or may all have differing values. For example, each capacitor may have a charging time that is equal to a unit amount of delay, d. Alternatively, the charging time of each capacitor may be controlled so as to have a binary equivalent. For example, the ‘a’ capacitors may have a charging time equal to the unit amount of delay, d. The ‘b’ capacitors may have a charging time equal to twice the unit amount of delay, 2d, or d+1. The ‘c’ capacitors may have a charging time equal to four times the unit amount of delay, 4d or d+3.

Turning now to the control circuit 50 of the SSCG, an embodiment is shown in FIG. 9. The control circuit 50 may comprise a frequency divider 80 for generating a lower frequency clock DFCLK and an address generator 82. The address generator may be implemented as a state machine, where the output of a new address signal causes the machine to change state to the next state. The number of addresses needed may be known, as the number of combinations of control code values, or control words, may be finite.

For example, there may only be four control ‘words’ used to activate the delay circuit. Four addresses, 1000, 0100, 0010, and 0001, may be used. An address generator to generate the addresses is shown in FIG. 10. When a reset signal, RESET, is applied, address signals A1-A4 are generated to 1000. A flip-flop generating address signal A1 generates a high signal in response to a set signal SE. Once the address signal A1 is generated, the high data of the A1 signal is shifted to a next address signal whenever the divided clock DFCLK is toggled. This results in address signals A1-A4, 0100, 0010, and, 0001. These are enabled in this sequence when a forward enable signal FCON is enabled.

After the final address A4 is activated (A1-A4 0001), a backward enabled signal BCON is enabled. This allows the high data of the A4 signal to be output in reverse order, A3, A2 and A1. Accordingly, the address signals A1-A4 are changed with order such as 0010, 0100 and 1000. The switches are either forward switches FSW1-FSW3, or backward switches BSW1-BSW3. This process of address generation is continuously repeated in order to generate address signals in response to the divided clock DFLCK. The value of the delay load can be changed with an edge variation as will be discussed with regard to FIG. 11.

In FIG. 11, the timing of the signals for address generation is shown. The reset signal initiates the process. The two clock signals, FCLK and the divided clock, DFCLK are also shown. In this particular embodiment, the DFCLK has a frequency that is half that of the fixed clock. Other frequency divisions may also be used.

The forward control and backwards control signals FCON and BCON are generated from the B2F and F2B signals as shown in FIG. 10. Their related timing signals are shown in FIG. 10. The resulting spread spectrum clock signal, SSCLK, has a delay associated with it. For example, the period T is the period of the fixed clock signal plus a unit of delay, d. The number of delay units added to the clock signal can be programmed to vary according to the desires of the system designer. In the example of FIG. 11, the period T+1 has a delay of d+1; the period T+2 has a delay d+3, and period T+1, has a delay of d+4. As the addresses cycle backwards, the delays also cycle backwards, as shown in FIG. 11.

The delays shown are determined by the control signals that reside at the addresses A1-A4. The below table shows the control signals C01, C02 and C03, in their control ‘words’ and their corresponding addresses. Referring back to FIG. 7, it can be seen that the addresses provided to the address circuit results in a particular control code being provided to the delay components as discussed above. An example of some control codes provided are shown in the below table. Control Signal Address CO1 CO2 CO3 0001 0 0 0 0010 1 0 0 0100 1 1 0 1000 0 0 1

This particular example assumes that there are 3 delay components as shown in FIGS. 8 a and 8 b. However, it must be noted that any number of delay components could be used, as well as any number of control codes. Further, the nature of the control codes themselves may vary. The control codes may be a binary representation of the delay, where a delay control code of 001 would result in a delay of 1, while a delay control code of 100 would result in a delay of 4.

Alternatively, the control codes may be equally weighted representations. The control code 100 may be a delay of 2. For example, the equally-weighted representations are included in the table below. Control Signal Address CO1 CO2 CO3 Binary Equal 0001 0 0 0 0 1 0010 1 0 0 4 2 0100 1 1 0 5 3 1000 0 0 1 1 4 In either case, the code may represent a number of repetitions of the delay.

In one embodiment, the register circuit could be eliminated, using the address as the control code. However, this removes one level of modularization that would otherwise provide more flexibility in the programmability of the delay circuit. For example, the register circuit could be reprogrammed or replaced with a new register circuit that would have different values for the predetermined addresses.

Suppose the delay associated with address 0001 was desired to be 4 instead of 0. Allowing removal or reprogramming of the existing register circuit having the control codes above is possible because of the register circuit being separated from the address generator. The register circuit could be any type of non-volatile memory such as an erasable electronically programmable read only memory (EEPROM), a fuse array, an electronically programmable read-only memory (EPROM), a read-only memory (ROM), etc.

Having illustrated and described the principles of embodiments of the invention, it should be readily apparent to those skilled in the art that the embodiments can be modified in arrangement and detail without departing from such principles. All modifications coming within the spirit and scope of the accompanying claims are claimed. 

1. A spread spectrum clock generator, comprising: a register circuit to store control codes corresponding to a predetermined delay; a delay circuit to receive a control code having a predetermined number of bits and to delay a fixed clock signal a period of time determined by the control code.
 2. The clock generator of claim 1, further comprising a control circuit receiving the fixed clock signal to generate addressing signals for the register circuit.
 3. The clock generator of claim 2, the register circuit comprises a non volatile memory.
 4. The clock generator of claim 2, the control circuit comprises a frequency divider receiving the fixed clock signal and an address generator receiving an output clock signal of the frequency divider.
 5. The clock generator of claim 4, the address generator comprises a counter or a shifter register
 6. The clock generator of claim 1, the delay circuit comprising an inverter to invert the fixed clock signal and a series of delay elements.
 7. The clock generator of claim 6, the delay circuit further comprising metal-oxide semiconductor capacitors electrically connected to an output of the inverter, wherein each capacitor receives one bit of the control code.
 8. The clock generator of claim 7, the capacitor comprising one of the group comprised of: PMOS capacitor, NMOS capacitor
 9. The clock generator of claim 7, the capacitors comprising an NMOS capacitor as a first load, and a PMOS capacitor as a second load.
 10. The clock generator of claim 6, the delay elements comprising an access transistor and a capacitor electrically connected to each bit of the control code.
 11. The clock generator of claim 7, each capacitor for each bit having a same capacitance value.
 12. The clock generator of claim 7, each capacitor for each bit having a capacitance different from capacitors for other bits.
 13. The clock generator of claim 1, the control code further comprising a binary-weighted value.
 14. The clock generator of claim 1, the control code further comprising an equally weighted value.
 15. The clock generator of claim 1, the control code further comprising a representation of a number of repetitions of a.
 16. The clock generator of claim 15, unit of delay the unit of delay being dependent upon an input frequency of the clock signal.
 17. A spread spectrum clock generator, comprising: a register circuit to store control codes, each control code corresponding to a predetermined delay; an address generator receiving a fixed clock signal to address the register circuit sequentially; and a delay circuit to receive a control code and to delay the fixed clock signal a delay period corresponding to the control code.
 18. The spread spectrum clock generator of claim 17, the register circuit comprising one selected from the group comprising: a fuse array, a read-only memory, an electrically erasable programmable read only memory, and an electrically programmable read only memory.
 19. The spread spectrum clock generator of claim 17, the address generator comprises a counter or a shift register.
 20. A spread spectrum clock generator, comprising: a register circuit to store control codes, each control code corresponding to a predetermined delay; an address generator to address the register circuit sequentially; and a delay circuit to receive a control code and to delay a fixed clock signal a delay period corresponding to the control code; a frequency divider receiving the fixed clock signal and applying a first clock signal to the address generator
 21. The spread spectrum clock generator of claim 20, the frequency of the first clock signal is lower than that of the fixed clock signal.
 22. The spread spectrum clock generator of claim 20, the control codes comprising binary-weighted values corresponding to different delay periods.
 23. The spread spectrum clock generator of claim 20, the control codes further comprising equally weighted values corresponding to different delay periods.
 24. A method, comprising: addressing a register circuit to access control codes; controlling a delay circuit with the control codes, such that each control code corresponds to a different level of delay; applying varying delay times to a fixed clock signal, producing a spread spectrum clock signal of time varied frequency.
 25. The method of claim 24, addressing a register circuit further comprising: applying a frequency divider to a fixed clock signal to produce a lower frequency clock signal than the fixed clock signal; and providing the lower frequency clock to an address generator; and generating addresses to be used to address the register circuit in a sequential fashion.
 26. The method of claim 24, controlling a delay circuit further comprising: receiving a control code at the delay circuit; and activating elements in the delay circuit corresponding to non-zero bits in the control code.
 27. The method of claim 24, controlling a delay circuit with the control codes, such that each control code corresponds to a different level of delay, wherein the different levels of delay are multiples of a unit level of delay.
 28. A memory system, comprising: a memory controller; a memory module, comprising: a spread spectrum clock generator, comprising: a control circuit receiving a fixed clock signal and outputting an address signal; a programmable delay circuit receiving the fixed clock signal and outputting a first clock signal having variable delay period in response to an control code; and a register circuit receiving the address signal to store codes for the programmable delay circuit and to output the control code; and a plurality of memory devices.
 29. A memory system, comprising: a memory controller receiving a spread spectrum clock signal; a clock generator, comprising: a clock source outputting a first clock signal having a constant clock period; a phase locked loop receiving the first clock signal and outputting a second clock signal having higher frequency than that of the first clock signal; a spread spectrum clock generator receiving the second clock signal and outputting the spread spectrum clock signal, comprising: a control circuit receiving the second clock signal and outputting an address signal; a programmable delay circuit receiving the second clock signal and outputting the spread spectrum clock signal having variable clock period in response to an control code; and a register circuit receiving the address signal to store codes for the programmable delay circuit and to output the control code; and a memory module receiving the spread spectrum clock signal comprising a plurality of memory devices. 